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On 4/21/2017 1:51 PM, Fred McKenzie wrote:
In article , rickman wrote: A kayaking friend was asking me about antennas for marine VHF radios. Someone recommended this type of antenna. http://www.amateurradio.bz/2m-70cm_v...e_antenna.html I guess my first question is what would this be dual band? Is it because it works over a range from 2 meters to 70 cm? Or is it somehow tuned for both bands at the same time but not a lot in between? This antenna would need to be vertically mounted, so would be a little clumsy sticking three foot up from a kayak. Light weight would be better than heavier. The construction plans call for stainless steel material. Any reason to not use aluminum? Is diameter important? To use aluminum I would want to use something thicker than 1/8 inch and likely hollow. I believe TV antennas are usually made from 1/4 inch tubing with maybe 1/16 inch thick walls. I guess that might make it harder to bend without kinking. Would it be ok to make it with some sort of couplers for the bends? Rick- I did not watch the video. I believe the antenna is "dual band" because it happens to also work on a frequency band that is three times the basic frequency. In other words, 146 and 440. That does not appear to make any difference with regard to the Marine VHF band. You would simply scale the dimensions by the ratio of the Two Meter frequency to the Marine band frequency, 146/156. The link is not a video. I appreciate the speculation, but I am researching this and am looking for facts. As others have pointed out, the design is basically a dipole but with J shaped elements 47.3 and 15.9 cm lengths. Someone else has suggested this is essentially a pair of dipoles of the two lengths. Usually a theoretical analysis can be found for any given antenna design, but I have not found anything other than construction projects for this design. They don't seem to cover theoretical aspects. That said, the antenna in the article does not appear to be the best choice for your application. The "J-Pole" antenna Jeff suggested would be my choice. Do some research on the J-Pole, but remember to design it for 156 instead of 146 MHz. Can you explain what would be preferable about the J-pole antenna? One big disadvantage is that it appears to be more like 6 foot long for 2 meter use. That could be rather heavy and clumsy on a kayak. -- Rick C |
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